Mendel's 1st law and it's explanation 

Mendel's 1^st law: In hybrid organisms, the factors (genes) responsible for the opposite characters remain side by side without being mixed, destroyed, and during germ cell formation, they are separated from each other and enter the different germ cells.

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Genetical Explanation of the Mendel's 1^st law:

We can take the fly under the name of Drosophila as an example to explain the law properly. For this fly,  L (dominant character) is responsible for the tall wings. Whereas, l (recessive character) is responsible for the short wings. Due to the cross between the two homozygous or characteristically pure Drosophila, we get all the children as to be tall (Ll) at F₁ generation. Because, the tall characteristic (L) is dominant over the short (l). But at the F₂ generation, we get back the short winged characteristic. That indicates that the short winged characteristic which was latent at the F₁ generation is not destroyed. Here at the F₂ generation the latent characteristic is back and the ratio between the tall and the short is 3:1. That means the three-fourth are tall and the one-fourth are short. And among the tall generations, 2 out of 3 is heterozygous (Ll) and rest one is homozygous (LL).